WASHINGTON — House Republicans today make their most concerted effort of the year to change federal abortion law with legislation that would ban almost all abortions after a fetus reaches the age of 20 weeks.

The “Pain-Capable Unborn Child Protection Act,” expected to pass by a comfortable margin late today, would be a direct challenge to the 1973 Roe v. Wade Supreme Court decision that legalized abortions up to the time a fetus becomes viable. Fetal viability is generally considered to be at least 24 weeks into the pregnancy.

The measure will be ignored by the Democratic-led Senate and the White House, saying the bill is “an assault on a woman’s right to choose,” has issued a veto threat.

Even if the policy were to become law, it would almost certainly face a legal challenge.

National Right to Life Committee legislative director Douglas Johnson said it was the “most significant piece of pro-life legislation to come before the House since the Partial-Birth Abortion Ban Act” that was enacted in 2003.

Rep. John Conyers of Michigan, top Democrat on the House Judiciary Committee, said the bill “clearly is an attack on women’s constitutional right to choose and is one of the most far-reaching bans on abortion this committee has ever considered.”

Some 11 state legislatures have passed similar measures.

Anti-abortion groups said the time frame in the House bill and other state laws, which ban abortion 20 weeks after conception, is equal to 22 weeks of pregnancy.

The sponsors of the bill also cited evidence — which opponents say is disputed — that fetuses can feel pain after five months.