WASHINGTON — Republicans and Democrats will put good will to the test when Congress returns this week to potentially incendiary fights over nominations, unresolved disputes over student loans and the farm bill, and the uncertainty of whether lawmakers have the political will to rewrite the nation’s immigration laws.

The cooperation evident in the Senate last month with passage of a bipartisan immigration bill could be wiped out immediately if Majority Leader Harry Reid, D-Nev., frustrated with GOP delaying tactics on judges and nominations, tries to change the Senate rules by scrapping the current three-fifths majority for a simple majority.

Republican leader Mitch McConnell of Kentucky has indicated it’s a decision Reid could regret if the GOP seizes Senate control in next year’s elections.

“Once the Senate definitively breaks the rules to change the rules, the pressure to respond in kind will be irresistible to future majorities,” McConnell said last month, looking ahead to 2014 when Democrats have to defend 21 seats to the GOP’s 14.

McConnell envisioned a long list of reversals from the Democratic agenda, from repealing President Barack Obama’s health care law to shipping radioactive nuclear waste to Yucca Mountain in Reid’s home state of Nevada.

Reid had served notice in April that the Democratic majority could change the Senate rules on “any given day,” and he was willing to do so if necessary.

Meanwhile, reflecting the will of the rank and file, House Speaker John Boehner of Ohio and other Republicans have said the comprehensive Senate immigration bill that couples the promise of citizenship for those living here unlawfully with increased border security is a nonstarter in the House.