The 17th Amendment

The 17th Amendment (ratified 1913) shifted the election of U.S. Senators from state legislatures to direct popular vote. This was a major change in federal-state relations and the democratic franchise.

Key effects:

Increased direct accountability of senators to voters rather than state legislatures.
Reduced the influence of state political machines and legislative bargaining on Senate composition.
Altered the balance of state-versus-federal influence in national lawmaking.

Critical perspective on federalism:

Many scholars and critics argue the 17th Amendment substantially weakened American federalism. Prior to 1913, state legislatures selected senators, which created an institutional check: states could influence national policy through their legislative choices. By transferring that selection power to the nationwide electorate, the amendment:

Removed an avenue through which states could counterbalance federal centralization. Senators no longer directly represented state legislatures’ preferences in Washington.
Brought senators into the same electoral incentives as House members, reducing a structural difference that previously encouraged attention to state interests.
Consolidated electoral pressure: because the same electorate chooses both representatives and senators, national political coalitions and party organizations have increased leverage over both chambers.

Consequences:

With senators responsive directly to popular national campaigns, the formal institutional mechanism that tied senators to state governments was weakened, and with it a check on federal aggrandizement.
The change also made it easier for national interests, parties, and well-funded campaigns to shape Senate composition, arguably diluting local or state-specific counterweights.

Sources & further reading:

The text of the Seventeenth Amendment
Scholarship on Progressive Era reforms and federalism

Sources & further reading:

U.S. Constitution, Seventeenth Amendment
Historical analysis of Progressive Era reforms