Challenges in each state have gained steam thanks to enthusiasm among the Democratic base, with some liberal activists and unions defying the Democratic Party establishment and bucking even the political operation of the White House.
In Arkansas, Democratic Lt. Gov. Bill Halter hopes to triumph against the more moderate Blanche Lincoln. And in Pennsylvania, longtime-Republican-turned-Democrat Arlen Specter is being challenged by Rep. Joe Sestak, whose primary challenge has helped make Specter a more reliable vote in the Democratic caucus.
Both Lincoln and Specter have amassed significant war chests to use in their defense.
According to the most recent campaign finance reports, Specter had $5.8 million in his campaign coffers as of April 28, and Lincoln had about $3.2 million for use in the closing weeks of her campaign.
Their financial support includes considerable support from their fellow lawmakers.
Leadership PACs have contributed more than $300,000 to Lincoln's reelection efforts, including gifts from about 60 percent of her fellow Democratic senators who have contributed since March of last year. Such giving ranks these interests among Lincoln's top supporters. She's also been a favorite of many of the industries with interests before the Senate Agriculture Committee, which she chairs.
Similarly, Specter has collected $172,000 from leadership PACs in support of his reelection bid, likewise ranking these interests among his top financial backers. While Specter had to return tens of thousands in campaign contributions from Republican lawmakers after he ditched the GOP in April of last year, more than two dozen of his Democratic compatriots in the Senate have generously donated to his campaign since his defection.
Lincoln and Specter have also had the support of some big guns.
President Barack Obama has endorsed both Lincoln and Specter. Obama's former campaign arm, now known as Organizing for America and run by the Democratic National Committee, has been making calls in Pennylvania on Specter's behalf, and the Democratic Senatorial Campaign Committee has been funding pro-Specter ads. Obama and Vice President Joe Biden have also cut advertisements in each race to protect the incumbents (for example, here and here).
Halter has raised more than $2.6 million in his quest. As of the most recent campaign finance report, he had $558,000 in cash on hand at the end of April.
Sestak, for his part, had raised about $3.6 million as of the most recent filings. On April 28, he reported nearly $3 million in his reserves. In May, he launched multiple TV spots critiquing Specter's record and Specter's previous allegiance to the Republican Party.
Online donors and support from the so-called "Netroots" has been touted in each race. MoveOn.org has endorsed both Sestak and Halter. In Pennsylvania alone, more than $882,600 flowed to Sestak through the liberal political action committee and fund-raising conduit ActBlue. Overall, contributions from Democratic/liberal groups rank head-and-shoulders above other interests as Sestak's largest funders, and Sestak ranks as the top recipient in the House from these interests and as the second highest recipient overall.
If such enthusiasm garners Sestak more votes than Specter in Tuesday's Democratic primary election, then he will unseat an incumbent who has served in the Senate since 1980.
Halter supporters, meanwhile, need more than just a simple majority of votes to defeat Lincoln. Arkansas election law requires the winner of the primary to secure more than 50 percent of the vote. Should neither Lincoln nor Halter win a majority of the vote, they would face off again in a June 8.
On the Republican side of the aisle in these states, voters will also go to the polls to select their party's Senate candidates.
In Arkansas, Republican Rep. John Boozman has raised about $1.2 million and emerged as the front-runner in a crowded field of candidates. Like Lincoln and Halter, if he's unable to secure a majority of the vote, the race would be decided in a run-off on June 8.
In Pennsylvania, former Republican Rep. Pat Toomey, who narrowly lost a primary challenge to Specter in 2004, stepped down as the head the conservative Club for Growth last year to run against Specter again. The potential of another tough Republican primary challenge was among the reasons Specter left the GOP. Toomey is expected to be victorious over other Republican candidates who have raised negligible sums compared to his $4.7 million war chest.