Washington ? While the Senate considers a 700-mile fence along the Mexican border, the House is going underground, working on legislation to crack down on those who would smuggle illegal immigrants and drugs through cross-border tunnels.

The tunnel bill was one of three border security measures the House was taking up today as part of the pre-election effort by congressional Republicans to show they are serious about stopping the flow of illegal immigrants across the nation’s porous borders.

“Securing our borders is a major step forward in addressing comprehensive immigration reform,” Senate Majority Leader Bill Frist, R-Tenn., said as he urged Congress to act to close the borders before moving to broader immigration reform.

The Senate in May gave strong support to legislation that would set up a guest worker program and outline a path by which the estimated 12 million illegal immigrants already in the country could work toward legal status and eventual citizenship.

But there’s been little progress in reaching a compromise with the House, which last December passed a bill that tightens the border and cracks down on undocumented workers but says nothing about opening a course for citizenship, which many House Republicans consider tantamount to amnesty.

President Bush supports a broader approach including guest worker and citizenship provisions. In an interview with CNN’s Wolf Blitzer on Wednesday, Bush said he would sign a fencebuilding bill as part of efforts to strengthen the border. But “I would view this as an interim step. I don’t view this as the final product. And I will keep urging people to have a comprehensive reform.”

On Wednesday the House passed, on a partisan 228-196 vote, legislation that would eventually require voters to show proof of citizenship. Republican supporters said it would stop immigrants from voting illegally.

Democrats said it would disenfranchise legal voters, particularly minorities, the poor and the elderly who would have difficulty coming up with documents to prove citizenship.

But with the midterm elections only seven weeks away and Congress slated to recess next week, GOP leaders have made border security, along with security-related bills on terrorist detainees and wiretapping, their most pressing business.

The House last week approved the bill, now before the Senate, that would build a 700-mile fence along one-third of the U.S.-Mexico border.

The three bills before the House today contain many of the same provisions of the larger border security bill passed last December. One possible strategy is to include these smaller bills in a 2007 Homeland Security spending bill the House and Senate are now negotiating.

The tunnel bill would criminalize the construction or use of unauthorized passageways under the border with a prison term of up to 20 years.

The other bills would make it easier to detain and deport noncitizen gang members and criminals, and clarify the authority of state and local law enforcement officials who volunteer to help in detaining illegal immigrants.