How many members of the US senate are up for reelection every two years?

Senators however, serve six-year terms and elections to the Senate are staggered over even years so that only about 1/3 of the Senate is up for reelection during any election.

Explanation:

A senator's term of office is six years and approximately one-third of the total membership of the Senate is elected every two years

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powers  are  denied  to the  national governmentin three distinct ways: some  powers, such as the  power  to levy duties on exports or prohibit the freedom of religion, speech, press, or assembly, are expressly  denied  to the  national government  in the  constitution.