Ms. Blackburn is a Tea Party and Trump stalwart, as are many Tennessee voters. She also represents a type of conservatism that may be peaking in some parts of the South: combative, inflexible and more interested in picking fights than actually governing. An aggregate of recent polls has Mr. Bredesen leading her by 5 percentage points.

Mr. Bredesen spent two terms as governor, from 2003 to 2011, with a pro-business reputation. But since he last ran for office, in 2006, when he won all 95 counties, his party has suffered a string of defeats: only in Nashville and Memphis do Democrats hold congressional seats; at the state level, Democrats have been reduced to superminority status in both houses, meaning they are not even needed for the Legislature to hold session.

Why then did Mr. Bredesen, at 74, dive back into the political fray? Associates say that he never would have challenged Mr. Corker, a friend and, like him, a former mayor and businessman, but the thought of losing a nominal check on Mr. Trump bothered him. And even last fall, before the Democrat Doug Jones beat Roy Moore for a Senate seat in Alabama, internal Democratic polls put him slightly ahead of Ms. Blackburn in a state that Donald Trump carried by 26 percentage points.

Ms. Blackburn, on the other hand, was ready made for the Trump era. As a state senator she hounded Mr. Sundquist on taxes; after jumping to Congress, she became an at-the-ready Obama critic who spent the last decade as a fixture on cable news outlets. She’s one of the president’s strongest defenders in Congress, voting with him 91 percent of the time.