Former President Obama on Wednesday sent well wishes to Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (R-Ariz.) after it was announced that McCain has been diagnosed with brain cancer.

"John McCain is an American hero and one of the bravest fighters I've ever known. Cancer doesn't know what it's up against. Give it hell, John," Obama tweeted.

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Mayo Clinic Hospital in Phoenix announced McCain's diagnosis on Wednesday.

The tumor was discovered after the senior Arizona senator underwent a minor procedure last week to remove a blood clot from above his left eye.

The news of McCain’s diagnosis prompted an immediate outpouring of support from fellow lawmakers, including Obama, Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellOcasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat McConnell locks down key GOP votes in Supreme Court fight Video shows NYC subway station renamed after Ruth Bader Ginsburg MORE (R-Ky.) and Sen. John Cornyn John CornynCalls grow for Biden to expand election map in final sprint Bipartisan praise pours in after Ginsburg's death Chamber of Commerce endorses McSally for reelection MORE (Texas), the No. 2 Senate Republican.