KENNETT, Mo. — By her own admission, state auditor Claire McCaskill did not spend enough time campaigning in rural Missouri two years ago, when she narrowly lost the governorship to Republican Matt Blunt.

This year, as the Democratic nominee against Senator Talent, a Republican, Ms. McCaskill is making up for lost time. She’s barnstorming across country roads in a bright-blue recreational vehicle, stopping in small towns such as this one, the home of singer Sheryl Crow.

Her ability to win votes in rural Missouri may determine whether she topples Mr. Talent in a neck-and-neck race. More broadly, it probably will signal whether Democrats can overcome the principal obstacle in their drive for a Senate majority: the Republican dominance of the Senate seats from “red” states that President Bush carried in 2000 and 2004.

From disenchantment over the Iraq war to the sex scandal surrounding Rep. Mark Foley, a Republican of Florida, a national tide may be gathering behind Democrats. For Republicans, the red states loom as the sea wall against that force.

Election Day will decide if discontent over the nation’s direction evident in polls overrides entrenched Republican advantages, especially with rural voters, in these culturally conservative states. One of the most powerful trends in American politics has been the growing alignment between the way states vote in presidential and Senate elections.

In the past, many states often would support one party in presidential races while sending members of the other party to Washington as one or both of their senators. But in a highly polarized era, more voters are supporting Senate candidates who come from the same party that they back for president.

Currently, the GOP holds three-fourths of the Senate seats from the 29 states that twice voted for Bush for president. Similarly, Democrats hold about three-quarters of the Senate seats from the 18 states that voted for Al Gore in 2000 and John Kerry in 2004.

Overall, Democrats need a net gain of six seats to win the Senate. Only two of their top targets year are Republicans in “blue” states that the Democratic presidential candidates carried in 2000 and 2004: Senators Chafee of Rhode Island and Santorum of Pennsylvania.

All of the other targets are in red states — incumbents Senators DeWine of Ohio, Burns of Montana, Allen of Virginia, and Kyl of Arizona, as well as Senator Talent of Missouri and an open seat in Tennessee.

Amid the bad political news that has battered the party this year, Republican strategists take comfort that so many of the critical Senate fights are being waged on what amounts to the GOP’s home turf. Still, the latest polls show the Democratic candidates running about even or slightly ahead of their Republican opponents in each of the hotly contested red-state Senate races except Arizona.

In most of these contests, Democrats express confidence that antipathy toward Bush will spark a large turnout by their core voters. They also believe concern about the Iraq war — as well as anger over other matters, capped by the Foley scandal — will improve their showing among relatively well-off, socially moderate suburban voters, like those who live near Kansas City or Cleveland or in northern Virginia.

In all but one of the key red-state Senate races, rural voters constitute a larger share of the population than nationally. Arizona is the exception. And that means to win the seats they need to control the Senate, Democrats likely will have to minimize the GOP edge among culturally conservative exurban and rural voters in places such as southern and western Ohio, eastern Tennessee and the southern sections of Virginia. Missouri precisely embodies this dynamic.

Experts in Missouri politics believe Ms. McCaskill is likely to post strong showings in Kansas City and St. Louis and their suburbs. But in her 2004 gubernatorial race, Ms. McCaskill won only eight of the state’s 109 rural and exurban counties, a St. Louis University political scientist Kenneth Warren, said. To win this year, he said, Ms. McCaskill’s main challenge “is to get more of the rural vote.”

Ms. McCaskill’s biggest obstacle, by far, is her liberal views on social issues: She supports abortion rights and opposed state constitutional amendments to ban gay marriage and allow residents to carry concealed weapons. Mr. Talent’s campaign has run ads in rural markets highlighting these positions.

Ms. McCaskill is trying to convince more rural voters to consider “the money side” of the race (in the Bootheel’s major counties, the poverty rate substantially exceeds the state average).

Mr. Talent, meanwhile, is attempting to localize the race by focusing on his legislative priorities, such as measures to require increased use of ethanol and to crack down on methamphetamine production.