By Charles Abbott

WASHINGTON, June 7 (Reuters) – Senator Blanche Lincoln faces possible elimination in a Democratic run-off primary in Arkansas on Tuesday, putting at risk her tough stance against the big banks in Wall Street reform.

Lieutenant Governor Bill Halter held a small lead over Lincoln in polls of likely voters ahead of the vote. Lincoln led in the May 18 primary but did not get an outright majority to claim nomination for a third Senate term.

Lincoln is the author of a provision to force banks to spin off their swaps desks, which potentially could cost them billions of dollars in revenue. Lincoln says the step would prevent taxpayer bail-outs of banks due to risky trades.

Because of anti-Washington sentiment, Lincoln is regarded as one of the most vulnerable incumbents in this year’s congressional elections. Opinion polls give Republican nominee John Boozman a huge lead for the general election in the fall.

Here are scenarios for the runoff election and its impact in Washington:

LINCOLN WINS THE NOMINATION

A victory would vindicate Lincoln as a centrist in tune with the political mainstream and elevate her stature in House-Senate negotiations expected to begin this week on financial reform. Her swaps-desk language ranks as one of the key issues to resolve.