run out of steam

run out of steam AND lose steam (to lose energy or desire to continue doing sth; lose momentum) — терять энтузиазм, терять запал; выдохнуться; сдуться (разг., сниж.); ~ опускать руки;
Also run out of gas {AmE}

Example: The politician ran out of gas during the campaign.

see also
[упасть духом]
[опускать руки]

Opp.
recharge one's batteries