WASHINGTON (Reuters) - A senior House Republican warned on Wednesday that differences among lawmakers over a spending increase could threaten plans for a more orderly budget process under House Speaker Paul Ryan, who has been basking in a honeymoon period. The comments from House Appropriations Chairman Hal Rogers came as Ryan was facing the first big challenge of his speakership, with some conservatives wanting to back out of a deal made last year increasing spending by $80 billion over two years. Many Republicans did not vote for the two-year budget framework negotiated in former Speaker John Boehner’s waning days in office. Ryan was elected to replace him late in October. Conservative veterans of past budget battles see little reason why they should now support a budget for fiscal 2017 that incorporates the spending hike. “The new number isn’t something that I could vote for unless I have a compelling reason to do so, and I haven’t been given that reason,” said Representative Matt Salmon, a member of the conservative House Freedom Caucus. Democrats are unlikely to welcome changes to the fiscal deal, especially cuts, and could leave Ryan only fellow-Republicans to negotiate with on a new plan. Rogers warned that if the House tries to change the deal, the Senate might balk, which could ultimately lead to the need to pass a massive “omnibus” spending bill to fund the entire government. “So we’ve got to consider carefully what we do here,” Rogers said after a Republican caucus. An omnibus would be the opposite of the better-functioning appropriations process Ryan and Senate Majority Leader Mitch McConnell, also a Republican, seek this election year. They want Congress to pass a dozen separate spending bills  instead of voting on one huge piece of legislation that, if it fails, results in a government shutdown. The first move would normally be for the House to pass a budget blueprint, which leaders hope to do soon. Rogers thought lawmakers could “deem” a budget resolution as having passed already because of last year’s deal. But such a procedural workaround could upset conservatives. Some Senate Republicans are also wondering where the votes will come from to pass a budget resolution embracing higher spending levels. Congress “already agreed (to the spending numbers) and did the tough choice” of raising spending, a senior Senate Republican aide said. “Why put our members who are in-cycle (up for re-election this year) through this again?” 