A French government plan to enshrine the "freedom" to have an abortion in the constitution is to be tested Wednesday in a crucial vote in the right-dominated Senate.

President Emmanuel Macron last year pledged to inscribe the right to terminate a pregnancy -- which has been legal in France since 1974 -- in the constitution after the US Supreme Court in 2022 overturned the half-century-old right to the procedure, allowing states to ban or curtail abortion.

France's lower-house National Assembly overwhelmingly voted in favour of making abortion a "guaranteed freedom" in late January, with almost all members of Macron's centrist minority coalition as well as left-wing opposition parties approving it.

But the plan still needs backing from the Senate, which is controlled by the right, with several leading senators opposed to the change.

Only if the upper house approves the government's exact wording on Wednesday afternoon can a combined vote of both chambers of parliament be held next month towards changing the constitution.

Otherwise it will be sent back to the National Assembly for further debate. 

The government chose the expression "guaranteed freedom" as an apparent compromise between both houses.

The lower house in 2022 had approved enshrining the "right" to an abortion, while the Senate last year was in favour of adding the "freedom" to resort to the procedure.

A Senate committee on Wednesday morning rejected motions from the right to amend the text of the proposed revision, several members of parliament said, in a possible indicator the vote could be positive.

Even if Senate leaders are against Macron's proposal, in private several right-wing senators have said they feel under pressure to approve it.

"If I vote against it, my daughters will no longer come for Christmas," said one female senator who asked to remain anonymous.

More than three quarters of right-wing Republicans in the National Assembly voted in favour of adding the freedom to the constitution last month.

A survey by French polling company IFOP in November 2022 found 86 percent of French people were in favour of making having an abortion a constitutional right.