WASHINGTON, Feb. 23 — Backers of a bipartisan bill that would shield firearms dealers and manufacturers from lawsuits are pushing for a vote this week in the Senate, injecting the volatile issue of gun control into this year's presidential and Congressional elections. The bill, which would grant legal immunity to gun makers and dealers so long as they did not sell defective weapons or violate any law, has the support of the White House and 55 sponsors in the Senate, including the Democratic and Republican leaders. It has already passed the House and could come up for consideration in the...