WASHINGTON — For President Barack Obama and congressional Democrats, this month’s budget battles brought about a remarkable period of party unity, a welcome change for the White House after a summer of disputes over possible military action in Syria, government spying programs and the president’s pick to lead the Federal Reserve.

But Democratic solidarity will face a tougher test during the broader budget talks following the reopening of the government and the increase of Treasury’s borrowing authority. While the prospect of a large-scale agreement is slim, Republicans will try to extract concessions from Obama on spending, deficit reduction and entitlement reform — all areas where Democratic lawmakers have worried the president is willing to give up too much.

“When things get serious, some of these negotiations are going to be awfully tough for people,” Jim Manley, former adviser to Senate Majority Leader Harry Reid, D-Nev., said of congressional Democrats.

Throughout the 16-day shutdown and march toward the debt ceiling deadline, congressional Democrats lined up solidly behind Obama and his vow to not negotiate with Republicans. It was a hard-line stance that many in the party wished he had taken during previous fiscal fights.

Democratic unity was further bolstered by the fissures that emerged among Republicans and a burst of polling that showed the GOP was taking the brunt of the public’s blame for the shutdown. And in the end, every congressional Democrat voted for the deal that keeps the government open until Jan. 15, lifts the debt ceiling until Feb. 7, and opens two months of budget negotiations.

But if any agreement does emerge from those talks, it will likely require Obama to make concessions that could rankle his Democratic allies.

The biggest sticking point could be over reforms to federal benefit programs, which Democrats have refused to accept without accompanying increases in tax revenue. The budget plan Obama outlined this year put Democrats on edge because it proposed bold changes to Medicare and Social Security.

One possible compromise in the end-of-the-year talks may involve Obama offering more modest entitlement changes in exchange for easing the automatic spending cuts known as the sequester.

The sequester is unpopular with both parties. But there is little consensus over how to offset the spending cuts, which are scheduled to intensify in mid-January, with the Pentagon bearing most of the cuts.