WASHINGTON (AP) - 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.

McConnell made no attempt to minimize his own interest in the

Senate primary in Kentucky after making a late television

commercial on behalf of Secretary of State Trey Grayson, battling

tea party-backed Rand Paul.

A spokesman, Don Stewart, said McConnell was watching the race

in his home state closely, and added he doubted White House claims

that Obama wasn't equally interested. "That sure would be a

surprise given that he has two incumbents in close races," he

said.

While Grayson had support from the state's Republican

establishment, Paul countered with backing from former Alaska Gov.

Sarah Palin, retiring Sen. Jim Bunning and conservative Sen. Jim

DeMint of South Carolina. DeMint has interceded in several

primaries in hopes of pushing his party to the right, a decision

that some Republicans say may portend a move for greater influence

inside the Republican leadership led by McConnell.

Among Democrats, Kentucky Attorney Gen. Jack Conway collided

with Lt. Gov. Daniel Mongiardo for the right to take on the

Republican winner. Mongiardo lost a close race to Bunning six years

ago.

Specter, 80 and a party-switcher, struggled for political

survival in a primary with Rep. Joe Sestak, who gained late

momentum with a television ad. It showed his rival saying only a

year ago that he quit the Republican Party to win a new term.

Former Rep. Pat Toomey campaigned as the prohibitive

front-runner for the Republican nomination, six years after losing

to Specter in a GOP primary.

In Arkansas, Lincoln sought renomination against Lt. Gov. Bill

Halter. She emphasized her independence from party. Halter had the

support of several unions that spent heavily in hopes of punishing

the incumbent for votes on health care, trade and legislation to

make it easier to organize workers.

The presence of a third contender on the ballot, D.C. Morrison,

raised the possibility that Lincoln might be forced into a

politically debilitating runoff on June 8.

Rep. John Boozman was the acknowledged Republican front-runner

for the Senate nomination for a seat the GOP hopes to win in the

fall.

Oregon's mail-in primary produced little if any of the drama

that was on display elsewhere.

Sen. Ron Wyden sought the Democratic nomination to a third full

term. Seven Republicans vied for the right to oppose him in the

fall.

Former Democratic Gov. John Kitzhaber campaigned for his party's

nomination for a return to office, and nine Republicans competed

for the nomination to run against him.

In Pennsylvania's gubernatorial primary, four Democrats and two

Republicans vied to advance to the fall election. Gov. Ed Rendell,

a two-term Democrat, was barred from seeking re-election.