House Democratic leaders are facing repeated challenges from their rank-and-file members, who are bucking them on procedural motions and other votes that usually fall along party lines.

That trend was exemplified last week when 18 members, including a member of the Democratic whip team and a committee chairman, defied House Speaker Nancy Pelosi (D-Calif.) by rejecting a motion to bring an ethics bill to a floor vote. The defections nearly brought down the bill, a key Pelosi priority.

Eight Democrats defected on the vote to bring the leadership’s surveillance bill to the floor, and Rep. Nick Lampson (D-Texas) — a GOP target this election year — wouldn’t vote to bring the Democrats’ budget to the floor.

Democrats have also challenged their leaders on substantive policy.

Four Democrats voted for the Republican budget plan. Nine have signed a GOP-backed “discharge petition” to bring a border security bill opposed by Pelosi to the floor. They are Reps. Heath Shuler (N.C.), Gene Taylor (Miss.), Brad Ellsworth (Ind.), John Barrow (Ga.), Lampson, Mike McIntyre (N.C.), Nancy Boyda (Kan.), Joe Donnelly (Ind.), and Chris Carney (Pa.).

The defections represent a marked contrast from the strict discipline enforced during Republican control of the House, but leadership aides say it doesn’t represent a problem because in each case, Democrats and their priorities prevailed.

The ethics bill, the Democratic budget, and the House Democrats’ surveillance plan all got to the floor and passed. The border security bill, they note, is still far from getting the 218 signatures needed to bring it to the floor, in large part because Republican leaders have so far failed to convince more than 25 of their own members to sign the discharge petition.

“We’ve been effective in passing our agenda,” said Kristie Greco, spokeswoman for House Majority Whip James Clyburn (D-S.C.). “They tried to put us in a political box and we haven’t taken it.”

The bucking of leadership also reflects the diversity of the Democratic Caucus, which includes a number of freshman lawmakers representing conservative districts. Democrats routinely describe this diversity as both a challenge and a virtue.

But not all senior Democrats are taking the defections in stride. House Appropriations Committee Chairman David Obey (D-Wis.) last week told members at an early-morning caucus that he’s not in the mood to do any favors for Democratic members who won’t vote with party leaders on procedural votes.

“I’m tired of people coming to me for help when they can’t even vote with us on a procedural matter,” Obey said in an interview with The Hill. “Even if some people in leadership don’t care, I do care.”

He made good on his threat by nixing a meeting set up by Rep. Charlie Melancon (D-La.), though he later backed down. Melancon was one of the 18 who voted against bringing the ethics bill to a roll call.

Asked about the other 17 defectors, Obey said, “They didn’t have anybody scheduled with me.”

The procedural fight has its roots in a strategic decision made by Republican leaders early last year, said Kevin Smith, spokesman for Minority Leader John Boehner (R-Ohio). He said they opted to use procedural votes, and particularly the “motion to recommit,” more aggressively than Democrats had in the minority, to “make vulnerable Democrats choose every day between Speaker Pelosi and their constituents.”

When Republicans were in power, they took a “hard line,” a GOP aide said, demanding party loyalty on nearly all votes, even from vulnerable members.

Republicans describe luring Democrats into a trap by initially offering non-controversial motions, such as banning the use of federal funds in lobbying, which passed unanimously. Once Democrats had voted for a Republican motion on policy grounds, they couldn’t reverse and vote against them, GOP officals said.

“Once you’ve voted with Republicans on procedural votes, then they’re all policy votes,” said Stuart Roy, a former aide to House Majority Leader Tom DeLay (R-Texas).

Since implementing the strategy, Democratic leaders have had to pull bills from the floor eight times because of a Republican procedural motion, according to statistics provided by Republican leadership offices.

Republicans have won 23 motions to recommit since Democrats took over. Only 12 Democratic motions to recommit passed during the 12 years of GOP control, according to Republican officials.

Most motions can be stripped later in the legislative process, but two have been enacted into law — a condemnation of MoveOn.org’s advertisement targeting Gen. David Petraeus and a motion blocking lawsuits against people who report potential terrorist activity.

Democrats have dealt with each procedural challenge on a case-by-case basis. Efforts to develop a consistent policy have not taken hold. Last year, leaders asked members to vote uniformly against recommit motions that would kill a bill, but vulnerable members rejected the policy when Republicans sought to add immigration language to an agriculture spending bill.

Rep. Brian Baird (D-Wash.), who voted against leadership on the ethics bill, said he’d like to see broad changes in how Democratic and Republican leaders handle procedural fights, such as allowing more time to review the motions from the minority.

“ ‘Gotcha’ amendments come with being in the majority. We’ve got to have the intestinal fortitude to deal with them,” Baird said. “But having 15 minutes to read them and vote does a disservice to the process.”