Party leaders are quickly wading into New Jersey’s Democratic incumbent-vs.-incumbent grudge match between Reps. Bill Pascrell and Steven Rothman, with Senate Majority Leader Harry Reid joining a growing number of top Democrats throwing their financial weight behind Pascrell’s campaign.

On Tuesday, the Nevada Democrat donated $5,000 to Pascrell’s campaign through his Searchlight Leadership Fund PAC, joining House Assistant Minority Leader James Clyburn (S.C.) and House Democratic Caucus Chairman John Larson (Conn.).

Larson kicked in $2,000 to Pascrell’s campaign Dec. 31, while Clyburn donated $7,000 total from his campaign fund and PAC earlier this month.

The two incumbents face off June 5 for supremacy over their northern New Jersey district.

Leadership of either party rarely wades into the middle of primary battles between incumbents, and in the past when they have, it has often had disastrous results.

House Majority Leader Eric Cantor (Va.) earlier this year touched off turmoil within his Conference when he backed Rep. Adam Kinzinger over Rep. Don Manzullo in their Illinois primary race. The move enraged Manzullo, who demanded Cantor step down from his leadership position.

Although Cantor’s bet paid off and Kinzinger won the contest, the episode left a sour taste among his colleagues, particularly old-line veterans of the House who saw the endorsement as a breach of protocol.

Both Clyburn and Larson insist their decision was based solely on personal considerations.

“It’s very, very personal. I grew up in South Carolina, [but] I spent my summers during my college days in Paterson, New Jersey,” Clyburn told Roll Call, noting that he and his wife were married there.

“Ever since Bill Pascrell came to Congress, we’ve spent almost every evening together,” he said.

As for any hurt feelings within the broader Caucus, Clyburn said simply “we’re very, very close personal friends. … I’ve got personal relationships, I hope everybody will understand that.”

A Larson campaign spokesman said the Connecticut Democrat was motivated by similar reasons. Pascrell is “a personal and dear friend, and friendship transcends politics,” the spokesman said.

Reid’s decision to extend his already considerable political reach not only into the House, but all the way into a New Jersey primary between two incumbents is more puzzling. He and Pascrell share no obvious personal or professional ties. A Reid spokesman declined to comment.

At the same time, rank-and-file Democrats are beginning to pick their own favorites. Rep. Henry Waxman (Calif.) donated $2,000 to Rothman, while Rep. Jesse Jackson Jr. (Ill.) donated $1,000 through his A More Perfect Union PAC. Rep. Ted Deutch (Fla.) donated $2,000 to Rothman.

So far, the two campaigns are taking the battle lines being drawn within the party in stride.

“Congressman Pascrell appreciates all the support he’s received from his colleagues in Congress in his primary fight, including the support of so many Democratic leaders. He’s very proud of the relationships he’s built with so many of his colleagues as they work together on behalf of middle-class taxpayers across the country in the fight against tea party extremists,” Pascrell spokesman Sean Darcy said Wednesday.

Likewise, Rothman spokesman Paul Swibinski said, “We welcome their support and thank them for their contribution.”

Abby Livingston contributed to this report.