Washington - The White House issued its second veto threat in as many days on Wednesday against top-priority legislation of the new, Republican-controlled Congress.Republican leaders, in turn, challenged President Barack Obama to sign measures that clear the House and Senate with support from at least some Democrats."We're calling on the president to ignore the voices of reaction and join us," Senate Majority Leader Mitch McConnell said as he and House Speaker John Boehner lined up legislation to approve the Keystone XL oil pipeline and make changes to the health care law they also have vowed to repeal.Lawmakers formally opened the new session of Congress this week with the Republicans in charge of both the Senate and the House of Representatives for the first time since Obama took office. The party's sweep of the November elections is certain to complicate the final two years of Obama's presidency as he seeks to cement his policies on health, the environment and immigration.The conflict comes at a time when the Democratic president and the two Republican congressional leaders have all stressed the opportunity for bipartisanship, and polls generally indicate the public wants divided government to produce compromise instead of gridlock.Political priceBy approving measures with bipartisan support - the pipeline legislation has well over 60 supporters from both parties in the 100-member Senate - it appears Republicans are trying to make the president pay at least a short-term political price if he makes good on his veto threats.Far larger and more partisan fights likely lie ahead, particularly if, as expected, Republicans attempt to seek large savings in government benefit programs as part of an attempt to balance the budget.One day after saying Obama would reject the pipeline bill, the White House said he would veto legislation to make a change in the health care law he signed into law four years ago.The law, Obama's signature domestic achievement, has extended insurance coverage to millions of Americans who previous lacked it. Republicans have criticized the law for requiring Americans to carry insurance or face penalties. They also warn that it will worsen health care in the US.The Republican measure would increase the definition of a full-time worker who must be offered health coverage at work to 40 hours from the current 30.Written veto threatIn a written veto threat, the administration said the Republican legislation "would significantly increase the deficit, reduce the number of Americans with employer-based health insurance coverage, and create incentives for employers to shift their employees to part-time work - causing the problem it intends to solve."Republicans argue the health law's 30-hour requirement is encouraging companies to cut workers' hours.On the Keystone pipeline, Republicans have allies within the Democratic ranks."It's just wrong. It's just not the way you do business," Democratic Senator Joe Manchin said after the White House announced it would veto the measure. "If this is the start of things, it is a sad beginning."An identical bill passed the House last year with support from 31 Democrats.The 1 897km project is proposed to go from Canada through Montana and South Dakota to Nebraska, where it would connect with existing pipelines to carry more than 800 000 barrels of crude oil a day to refineries along the Texas Gulf Coast.