A push to amend the US Constitution failed in the Senate Thursday, as Democrats hoping to restrain the role of money in politics could not overcome Republican resistance.

The amendment idea won’t die with the Senate vote. Backers say that after US Supreme Court decisions – including Citizens United in 2010 – that affirm the right of people and corporations to pour unlimited funds into politics, it’s necessary to alter the Constitution to give Congress new power over campaign finance.

But the amendment’s defeat this week shows how hard it will be to succeed in such a drive.

Amendments launched in Congress must be approved by a two-thirds majority in each chamber. (This effort ended as Democratic sponsors failed to even clear the 60-vote hurdle to end Senate debate.) Then, rather than heading to the president for signature as an ordinary bill would, an amendment must be ratified by three-quarters of the states.

The final vote was 54 senators for ending debate and allowing the idea to come to a vote, and 42 against ending debate.

“I am extremely disappointed that not one Republican voted today to stop billionaires from buying elections and undermining American democracy,” Sen. Bernard Sanders, a Vermont independent who aligns with Democrats on this issue, said in a statement after the vote. “The fight to overturn Citizens United must continue at the grassroots level in every state in this country.”

Part of the challenge is that, because the Supreme Court has given wide berth to the notion that money is a constitutionally protected form of speech, the amendment effort inherently involves modifying the hallowed First Amendment right to free speech.

At present, the two major parties remain far apart on whether that’s an acceptable step.

Democrats generally say a constitutional change wouldn’t restrain basic free-speech rights – and would in fact enhance them by making the power of speech less dependent on how much money one has. Many Republicans argue that tampering with the Bill of Rights would create a slippery slope by which a future Congress might undermine free-speech rights.

“I have to say it’s a little disconcerting to see the Democrat-led Senate focusing on things like reducing free-speech protections for the American people," Senate minority leader Mitch McConnell (R) of Kentucky said before the vote.

Earlier this week, in floor debate, Sen. Ted Cruz (R) of Texas lamented that not a single Democrat had spoken out “against this abominable provision,” which he called a “massive intrusion on civil liberties.”

Limits by Congress on direct contributions to candidates and political parties remain intact. But Supreme Court rulings such as the Citizens United case in 2010 have opened the door for independent groups – funded by corporations or wealthy donors – to spend unlimited sums on ads that seek to influence election outcomes.

In many cases, donors are also able to hide their identities, so voters may never find out precisely who is funding specific political messages.

Critics of the current campaign-finance climate say it is making members of Congress more beholden to special-interest donors than to the people they’re supposed to serve.

Sen. Michael Bennet (D) of Colorado said approving the amendment would help restore a less polarized Congress.

“Because of the new world of unlimited spending, Members of Congress are a lot less likely to seek compromise than they once were,” he said in a statement released after the vote. “We will continue to fight to bring sanity back to our campaign system.”

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So far in the current election cycle, nearly half of some $177 million spent on ads for Senate races across the country has come from outside groups rather than candidate- or party-raised funds, according to the Center for Public Integrity.

The proposed amendment that Democrats brought to the floor read, “Congress shall have power to regulate the raising and spending of money and in-kind equivalents with respect to Federal elections,” and it would have given states similar authority to regulate money in state elections.