The deadline is 5 p.m. Tuesday.
After that, the races should be set for the Aug. 7 primary election - unless the Missouri Supreme Court overturns the new district lines drawn last year for Missouri's U.S. House seats and for the Legislature's House of Representatives.
The district lines had to be redrawn after the 2010 U.S. Census, to ensure each district has equal population based on the census numbers.
The Legislature drew the congressional districts last year. Gov. Jay Nixon vetoed them, but lawmakers overrode the veto.
After a residents commission failed to draw new state House districts, a panel of six appeals court judges drew the map.
Both maps were challenged in lawsuits, with the high court hearing arguments in both cases last month.
However, the court has not issued any decisions, yet.
Ryan Hobart, Secretary of State Robin Carnahan's spokesman, said Friday candidates are supposed to be filing under the districts as drawn - they're considered official unless the court orders them to be changed.
Winners of the August primary, and those who faced no primary opposition, then go on to the Nov. 6 general election and voters' ultimate choice for the next few years.
Candidates for federal and state offices - including circuit court judges - must register in person at the secretary of state's office, on the third floor of the Kirkpatrick Information Center, 600 W. Main St.