As a very close person to the newly appointed Secretary-General of the PSOE, Pedro Sánchez, she served as Secretary for Local Policy of the Spanish Socialist Workers' Party.

After the 2016 general election, the possibly of a third consecutive election was imminent and the Secretary-General Sánchez proposed to the Federal Committee of the PSOE (the executive body of the Party) the possibility of holding an Extraordinary Congress with an urgent nature to decide if they should allow the Second Rajoy Government or rejecting it (the position of Sánchez) by asking the voters.

The position of most of the members of the Federal Committee was to refrain and allow Mariano Rajoy to form a new government, rejecting Sánchez's request and forcing his resignation.

[4] She was one of the decisive supports in the decision of Pedro Sánchez to attend the 2017 leadership election to regain the leadership of the PSOE and played an essential role in the campaign of the candidate, based on the rejection of the ideological project of the People's Party and the importance of the socialist bases to recover the party.

[5] After the success of the 2018 vote of no confidence in the government of Mariano Rajoy, Pedro Sánchez was elected Prime Minister of Spain and the Leader (Spokesperson) of the Socialist Group in the Congress of Deputies Margarita Robles was appointed Minister of Defence, promoting Lastra to that position, becoming the youngest person and the third woman to occupy this position in the history of the Socialist Party.