Two dozen House conservatives are threatening to oppose a sweeping defense policy bill over a nonbinding provision aimed at allowing immigrants brought illegally to this country as children to serve in the military.

Their move could provoke a clash on a perennially volatile topic that’s become a major issue in the presidential campaign.

In a letter to the chairman of the House Rules Committee, the lawmakers led by Rep. Mo Brooks, R-Ala., argued that the House had already voted three times to declare the Obama administration acted unconstitutionally when it granted deportation relief to immigrants brought here illegally as children.

They said that the amendment on military service by Rep. Ruben Gallego, D-Ariz., “contradicts the House’s previous position and is a severe threat to passage” of the $612 billion defense policy bill.

“Especially in this time of increased terrorism, our national security should not be threatened by allowing such controversial language on a program we have rejected three times as unconstitutional,” they wrote.

Gallego’s amendment was included in the defense bill during committee consideration last week, winning support from six committee Republicans. It expresses the sense of the House that the Pentagon should review the law on who can serve in the military, in order to determine whether it should include immigrants who’ve gotten work permits and deportation relief under the Obama administration’s Deferred Action for Childhood Arrivals program.

The bill is expected to be taken up by the full House next week. Brooks and the 24 other lawmakers who signed the letter said that they would offer an amendment to strike Gallego’s language.

A spokesman for House Speaker John Boehner declined to comment. But the annual defense policy bill typically passes with broad bipartisan support, and opposition from Brooks and his allies would not likely be sufficient to prevent its passage.

In the Senate, Armed Services Chairman John McCain, R-Ariz., rejected the idea of including a similar provision in his version of the bill, which is expected to be approved by his committee later this month. He said that “we have too full a plate” with issues related specifically to defense.

“Let’s have a debate on it, let’s have a discussion, let’s have it stand alone, rather than trying to put it in the defense authorization bill which could kill the bill,” McCain said.

The immigration issue gained prominence in the presidential campaign this week as Hillary Rodham Clinton, seeking the Democratic nomination, announced her support for creating a path to citizenship for immigrants in the country illegally. It amounted to a direct challenge to Republicans, most notably the party’s as-yet undeclared front-runner, Jeb Bush, who has endorsed creating a form of legal status for unauthorized immigrants — but stops short of saying they should be granted citizenship.