President Barack Obama is not on the ballot in this week's primaries, nor is Sen. Mitch McConnell of Kentucky, the Republican Senate leader.

But both have a stake in intensely competitive Senate races in three states, contests testing the strength of the tea party among Kentucky Republicans and the durability of incumbent Democratic Sens. Blanche Lincoln in Arkansas and Arlen Specter in Pennsylvania.

In a fourth race of national significance, Republican Tim Burns and Democrat Mark Critz battled to fill out the term of the late Democratic Rep. John Murtha in a congressional district in southwestern Pennsylvania. Both political parties reported spending roughly $1 million to sway the race, turning it into a laboratory for the fall campaign, when all 435 House seats will be on the ballot.

Oregon voters also faced a deadline for returning ballots in a statewide mail-in vote that began more than two weeks ago.

On the eve of the busiest primary night of the year so far, White House spokesman Robert Gibbs said Monday that Obama was following the races, but "not that closely."

"We have supported incumbent Democratic senators and we've done a lot on behalf of each campaign," he added, referring to Lincoln and Specter.

Gibbs said Obama's involvement had not become an issue in the primaries.

Nor was it clear what impact his involvement would mean for the incumbents, under extraordinary political pressure in a year of well-documented voter dissatisfaction with Washington.

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