Missouri voters participate in a non-binding presidential primary today (Tuesday).
They are former House Speaker Newt Gingrich, Texas Rep. Ron Paul, former Massachusetts Gov. Mitt Romney and former Pennsylvania Sen. Rick Santorum. Gingrich does not appear on the Missouri ballot. Other candidates on the ballot, including those who have abandoned the race, are Gary Johnson, Herman Cain, Michael Meehan, Rick Perry, Keith Drummond, Jon Huntsman and Michelle Bachmann.
Here's a look in brief at where the 2012 Republican candidates stand on a selection of issues.
Gingrich: Platform calls for conservative judges and no subsidies for abortion but not a constitutional abortion ban.
Paul: Says federal government should have no authority either to legalize or ban abortion.
Romney: Says Roe v. Wade should be reversed by a future Supreme Court and states should decide their own abortion laws.
Santorum: Favors constitutional abortion ban and opposes abortion, including in cases of rape.
Gingrich: As House speaker in 1990s, engineered passage of a seven-year balancedbudget plan. It was vetoed but helped form a bipartisan balanced budget later. Paul: Would eviscerate federal government, slashing nearly half its spending, shut five Cabinet-level agencies, end spending on existing conflicts and on foreign aid. Romney: Defended financial sector bailout, criticized GM and Chrysler bailout. Cap federal spending at 20 percent of GDP. Santorum: Freeze social and military spending for five years to cut $5 trillion from federal budgets.
Gingrich: Repeal the financial industry regulations that followed the Wall Street meltdown. Restrict the Fed's power to set interest rates artificially low. Paul: Return to the gold standard, eliminate the Federal Reserve, eliminate most federal regulations. Romney: Lower taxes, less regulation, balanced budget, more trade deals to spur growth. Replace jobless benefits with unemployment savings accounts. Repeal new financial-industry regulations. Santorum: Eliminate corporate taxes for manufacturers, drill for more oil and gas, and slash regulations.
Gingrich: Shrink Education Department. But supported Obama administration's $4 billion Race to the Top grant competition for states.
Paul: Abolish the Education Department and end the federal role in education.
Romney: Supported No Child Left Behind law. Once favored shutting Education Department, later saw its value in "holding down the interests of the teachers' unions."
Santorum: Voted for No Child Left Behind law, now regrets vote. Wants "significantly" smaller Education Department but not its elimination.
Gingrich: Let oil and natural gas industries drill offshore reserves now blocked from development, end restrictions on Western oil shale development.
Paul: Remove restrictions on drilling, coal and nuclear power, eliminate gasoline tax, provide tax credits for alternative fuel technology.
Romney: Supports drilling in the Gulf, the outer continental shelves, Western lands, the Arctic National Wildlife Refuge and offshore Alaska; and exploitation of shale oil deposits.
Santorum: Favors drilling in the Arctic National Wildlife Refuge, scaling back "oppressive regulation" hindering drilling elsewhere, and eliminating energy subsidies in four years.
Gingrich: Convert EPA into "environmental solutions agency" devoted to research and "more energy, more jobs and a better environment simultaneously." Once backed tougher environmental regulation.
Paul: Previously said human activity "probably does" contribute to global warming; now calls such science a "hoax." Says emission standards should be set by states or regions.
Romney: Acknowledged that humans contribute to global warming, but later said "we don't know what's causing climate change." Cap and trade would "rocket energy prices."
Santorum: The science establishing human activity as a likely contributor to global warming is "patently absurd" and "junk science."
Gingrich: If the Defense of Marriage Act fails, "you have no choice except a constitutional amendment" to ban gay marriage.
Paul: Decisions on legalizing or prohibiting gay marriage should be left to states.
Romney: Favors constitutional amendment to ban gay marriage, says policy should be set federally, not by states.
Santorum: Supports constitutional ban on same-sex marriage, not leaving decision to states. "We can't have 50 marriage laws."
All would seek repeal of Obama's health care law.
Gingrich: Prohibit insurers from cancelling or charging hefty increases to insurance holders who get sick. Offer "generous" tax credit to help buy insurance. Previously supported mandatory coverage.
Paul: Opposes compulsory insurance and all federal subsidies for coverage.
Romney: Opposes federal mandate to obtain coverage; introduced mandate in Massachusetts. Proposes "generous" subsidies to help future retirees buy private insurance instead of going on Medicare.
Santorum: Would seek to starve Obama's health care law of money needed to implement it. Supported Bush administration's prescription drug program for the elderly, now regrets doing so.
Gingrich: In contrast to most rivals, supports option of giving legal status to illegal immigrants with deep roots in the U.S. and who have lived otherwise lawfully. Supports path to citizenship for illegal immigrants' children who perform U.S. military service. Make English the official language. Divert more Homeland Security assets to at Mexican border.
Paul: Do "whatever it takes" to secure the border, end right to citizenship of U.S.-born children of illegal immigrants, no social services for illegal immigrants, aggressive deportation.
Romney: Would veto legislation that seeks to award legal status to some young illegal immigrants who attend college or serve in the armed forces. Favors complete U.S.-Mexico border fence, opposes education benefits to illegal immigrants.
Santorum: Supports complete border fence, opposes education benefits to illegal immigrants.
Gingrich: Give younger workers the option of diverting Social Security taxes to private retirement accounts.
Paul: Says younger workers should be able to opt out of Social Security taxes and retirement benefits; benefits for today's retirees should be protected.
Romney: Starting with workers now under 55, raise age to qualify for full benefits, and limit inflation increases for wealthier beneficiaries. Protect status quo for people 55 and older.
Santorum: Proposes immediate steps to lower benefits for wealthier retirees, raise the age to qualify for full benefits and restrict inflation increases in benefits, both for current and future retirees. Supports option of private retirement accounts.
All support eliminating the estate tax and keeping Bushera tax cuts.
Gingrich: Choice of filing under current system or paying a 15 percent tax, preserving mortgage interest and charitable deductions. Cut corporate tax to 12.5 percent.
Paul: Eliminate the federal income tax and the IRS, and defund close to half the government.
Romney: No one with adjusted gross income under $200,000 should be taxed on interest, dividends or capital gains. Cut corporate tax rate to 25 percent.
Santorum: Triple the personal exemption for dependent children, reduce the number of tax brackets to two - 10 percent and 28 percent, exempt domestic manufacturers from the corporate tax and halve the top rate for other business.
Gingrich: Supports extending and strengthening investigative powers of Patriot Act. Supports continued use of Guantanamo Bay detention for suspected terrorists. Supported creation of Homeland Security apparatus. In 2009, said of waterboarding: "It's not something we should do."
Paul: Opposes Patriot Act as an infringement on liberty. Says terrorists would not be motivated to attack America if the U.S. ended its military presence abroad. Says: "Waterboarding is torture. And it's illegal under international law and under our law. It's also immoral."
Romney: No constitutional rights for foreign terrorism suspects. Campaign says he does not consider waterboarding to be torture.
Santorum: Defends creation of Homeland Security Department. Voted to reauthorize Patriot Act. Says airport screeners should employ profiling; "Muslims would be someone you'd look at, absolutely." Supports continued use of Guantanamo Bay detention but says Americans accused of being enemy combatants should have right to challenge indefinite detention in court. Says waterboarding has proved effective.
Gingrich: Supported Iraq war and opposed early withdrawal. Said U.S. forces should not have been used in Libya campaign, after he had called for such intervention. Opposes "precipitous" pullout from Afghanistan.
Paul: Bring most or all troops home from foreign posts "as quick as the ships could get there." Opposed U.S. intervention in Libya. Cut Pentagon budget.
Romney: Has not specified the troop numbers behind pledge to ensure the "force level necessary to secure our gains and complete our mission successfully" in Afghanistan.
Santorum: Says he would order bombing of Iran's nuclear facilities unless they were opened for international arms inspectors. Proposes freezing defense spending for five years.