BELFAST (Reuters) - A dispute between Boeing Co (BA.N) and Canadian rival Bombardier (BBDb.TO) that risks thousands of jobs in Northern Ireland could impact peace in the region, the leaders of its two main parties warned U.S. Vice President Mike Pence.  British Prime Minister Theresa May has asked President Donald Trump to urge the world s largest aerospace company to drop its challenge against Bombardier, which could endanger a factory that employs 4,500 people in the British province. Bombardier is Northern Ireland s largest manufacturing employer and May s Conservatives are dependent on the support of the small Northern Irish Democratic Unionist Party (DUP) for their majority in parliament.  For a small economy such as ours, the significance of the contribution that Bombardier makes cannot be understated. The threat facing us as a result of the ongoing case is alarming, and goes much wider than it may immediately appear,  DUP leader Arlene Foster and Michelle O Neill of Sinn Fein said in letter to Pence, a copy of which was seen by Reuters.  The security of our economy has and continues to be a crucial part of our efforts in delivering peace through prosperity. At a time when we are striving to take the next steps in our work on the Peace Process, and resolve our current political difficulties, this issue creates a new and potentially critical factor.  Irish nationalists Sinn Fein and the pro-British DUP have for months tried in vain to re-establish a devolved power-sharing government, a key part of the 1998 peace deal that ended 30 years of sectarian violence in Northern Ireland. Washington played a key role in helping broker the deal. The parties confirmed that their leaders sent the letter, dated September 12.   Boeing this year asked the U.S. Commerce Department to investigate alleged subsidies and unfair pricing at Bombardier, accusing it of having sold 75 of its CSeries medium-range airliners to Delta Air Lines (DAL.N) at well below cost price. A U.S. trade court is due to give a preliminary ruling on Boeing s complaint on Sept. 25. 