As budget negotiators in the U.S. Congress tried to close a deal on Tuesday that would avoid a Jan. 15 government shutdown, conservative groups were lining up in opposition to the measure, anticipating an uptick in federal spending.

Democratic Senator Patty Murray and Republican Representative Paul Ryan have been meeting in private for weeks in an attempt to sketch out overall federal spending for the rest of the fiscal year that began on Oct. 1 and for the one that begins on Oct. 1, 2014.

Conservative groups hold sway with House Republicans and with the 2014 midterm elections coming into focus, their opposition to the deal could complicate its passage. In some cases, the groups are backing more-conservative primary challengers to Republican incumbents they view as too moderate.

According to congressional aides, any tentative budget deal might allow spending to rise from the scheduled $967 billion for fiscal 2014 to around $1 trillion.

While that increase in outlays would be offset by raising some government fees and possibly cutting federal workers' retirement benefits, conservatives were rallying against the deal, even before it was reached.

Americans for Prosperity, which supports cutting taxes and government spending, called on congressional Republicans to "stand firm" in upholding a second round of across-the-board automatic spending cuts, which are scheduled to start in January.

"Otherwise, congressional Republicans are joining liberal Democrats in breaking their word to the American people to finally begin reining in government over-spending that has left us over $17 trillion in debt," said AFP President Tim Phillips.

If a Ryan-Murray deal is reached, it is expected to relax some of those cuts, which are known as "sequestration."

Late on Monday, another conservative group, Heritage Action for America, an offshoot of the Heritage Foundation, announced that it also could not support the emerging budget deal.

The group complained that such a deal would increase spending "in the near-term for promises of woefully inadequate long-term reductions."

Club for Growth is waiting for the content of any deal before it passes judgment, said Barney Keller, a spokesman for the influential conservative group. Club for Growth has opposed past efforts to replace sequester that involved delaying the cuts.

From the other end of the political spectrum, Democratic lawmakers have voiced opposition to any deal that takes a whack at federal workers' pensions without asking sacrifices from wealthier Americans.

Democrats also have been pushing for an extension of federal benefits for the long-term unemployed, which are set to expire at the end of this month. Republicans have voiced opposition to the extension, citing a falling jobless rate.

If Murray and Ryan manage to agree on a budget plan, it will be presented to members of the Democratic-controlled Senate and Republican-led House of Representatives. A House vote could come before Friday, when that chamber plans to recess for the year.

If it passes the House, the Senate is likely to vote on it late this week or next week.

Senate Democrats are banking on winning the support of at least five Republicans who want to ease the Pentagon's automatic spending cuts and avoid a budget standoff like last October's, which led to a 16-day government shutdown.

Failure to craft and pass a budget deal could lead to another such shutdown on Jan. 15, when current funds for most federal agencies are set to run out.